How Does a Transient Amorphous Precursor Template Crystallization

Abstract

Crystallization through metastable phases, such as polymorphism, plays an important role in chemical manufacture, biomineralization, and protein crystallization. However, the kinetics creating the final stable crystalline phase from metastable phases has so far remained unclear. In this study, crystallization via an amorphous precursor, the so-called multistep crystallization (MSC), is studied quantitatively in a colloidal model system. In MSC, amorphous dense droplets are first nucleated from the mother phase. Subsequently, a few unstable subcrystalline nuclei can be created simultaneously by fluctuation from the tiny dense droplets, which is different from previous theoretical predictions. It is necessary for these crystalline nuclei to reach a critical size N*(crys) to become stable. However, in contrast to subcrystalline nuclei, a stable mature crystalline nucleus is not created by fluctuation but by coalescence of subcrystalline nuclei, which is unexpected. To accommodate a mature crystalline nucleus larger than the critical size N*(crys), the dense droplets have to first acquire a critical size N*. This implies that only a fraction of amorphous dense droplets can serve as a precursor of crystal nucleation. As an outcome, the overall nucleation rate of the crystalline phase is, to a large extent, determined by the nucleation rate of crystals in the dense droplets, which is much lower than the previous theoretical expectation. Furthermore, it is surprising to see that MSC will promote the production of defect-free crystals. The knowledge acquired in this study will also significantly advance our understandings in polymorphism related processes.

Phase-field crystal modelling of crystal nucleation, heteroepitaxy and patterning

“…Summarizing, at low thermodynamic driving forces the crystalline phase nucleates directly from the non-equilibrium liquid (as observed in some of the colloid experiments [15]), while at large driving forces an amorphous precursor phase appears (as has been seen in other colloidal systems [16−18]). Further work is needed, however, to clarify whether the present model is able to reproduce crystal nucleation inside amorphous particle rafts floating in the liquid phase, as observed in [16].…”

Nonclassical pathways of crystallization in colloidal systems

“…These droplets are usually formed due to the presence of a metastable phase separation [36][37][38]. In colloidal systems, the process has been studied both experimentally [39,40], and theoretically [35,[41][42][43][44]. The number and variety of solutions that have been found to crystallize from dense liquid precursors lead to the idea that two-step crystallization pathways are quite universal crystallization processes.…”

Role of clusters in nonclassical nucleation and growth of protein crystals

“…9-11). An obvious situation where CNT will have limited applicability is in cases where the old and the new phases differ by at least two order parameters, e.g., density and structure (12).Recent theoretical, computational, and experimental efforts have demonstrated that densification and local increase in crystallinity need not occur simultaneously (13)(14)(15)(16)(17)(18)(19)(20). These results have inspired the development of a new approach that considers nucleation from solution as a multistep process attributing key roles to metastable intermediate states, coined "multistep nucleation theory" (MNT).…”